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Basic Transistor Series Regulator and Troubleshooting Power Supply

Linear and switching regulators are the two fundamental classes of voltage regulator. Both are available in an integrated circuit form. There are two basic types of linear regulator. One is the series regulator, and the other one is the shunt regulator. 

A simple representation of a series-type linear regulator and its basic components  control element is in series with the load between input and output. The output sample circuit senses a change in the output voltage. The error detector compares the sample voltage with a reference voltage. The error detectorcompares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.

Regulating action:

The following diagram shows a basic op-amp (comparator) series regulator circuit. Its simplified operation is as follows: The resistive voltage divider formed by R2 and R3 "senses" any change in the output because the voltage of the voltage divider changes when Vout is modified. When the output decreases because of a decrease in Vin or because of an increase in IL, a proportional voltage decrease is applied to the op-amp's inverting input by the voltage divider. Since the Zener diode holds the other op-amp's input at a nearly fixed reference voltage Vref, a small difference voltage (error voltage) is developed across the op-amp's inputs. This error is amplified and the op-amp's output voltage increases. This increase is then applied to the base of T1, causing the emitter voltage Vout to increase until the voltage at the inverting input again equals the reference (Zener) voltage. This action offsets the decrease in output voltage, thus keeping it nearly constant. T1 is a power transistor and is often used with a heat sink because it must handle all of the load current.

When Vin or RL decreases: Vout and Vfb tend to decrease, thus compensating for the attempted decrease in Vout and stabilizing the circuit.

When Vin or RL increases: Vout tends to increase. The feedback voltage Vfb also tends to increase and as a result, the op-amp's output voltage Vb1 - which is also the base voltage of control transistor T1 - tends to decrease, thus compensating for the attempted increase in Vout.

This cycle is repeated, keeping the output stabilized.

basic-op-amp-comparator-series-regulator-circuit.jpg

Practical Considerations: 

  • If an excessive amount of load current is drawn, the series power transistor can be quickly damaged or destroyed. Most regulators employ overcurrent protection in the form of a current limiter.

  • In the following experiments, a transistor instead of an operational amplifier will be used as a comparator and perform the duties of a regulator, but in a very similar way to the op-amp. A voltage divider acting as a "sample circuit" feeds the regulating transistor's base, the Zener diode is located at the emitter, and the controlling power transistor's base at the collector. You will be able to better understand how the circuit works in the following experiments.

  • For the emitter-follower transistor T1, a high-gain power transistor consisting internally of a Darlington pair is an ideal choice.

Troubleshooting Power Supplies:

 

Proposed now is a series of steps to troubleshoot power supplies. Even without actually turning on a power supply, you might find up to 80% of the problems causing the fault.

  1. Examine the circuit thoroughly, looking for possible errors like:

  • open conductors

  • open fuses

  • shorts

  • bad solder joints

and signs of overheating such as discoloured components or circuit boards, melted solder joints, etc.

 

     2. Check dubious areas for continuity with an ohm-meter

ohm-meter.jpg

    3. When it comes to applying power to the board, first make sure to have a good schematic diagram of the circuit. Using this scheme as a guide, apply power to the board and make sure all the voltages appear where they should; this necessitates a thorough analysis of the schematic diagram.

troubleshooting-power-supply.jpg

     4. Once you have gotten to the stage of applying power and performing measurements at different parts of the circuit, look at the reference voltage and divider network. A properly functioning power supply just regulates to the reference voltage or a ratio of it. If a wrong reference voltage is adjusted, the output signal will naturally be wrong too.

 

     5. If one or more outputs are out of tolerance or exhibit excessive ripple at the line frequency or twice the line frequency, look for a dried main filter capacitor at the rectified AC input.

 

     6. If you notice periodic power cycling, sounds or blinking power lights, the semiconductors might be shorted or the controller element might be faulty.

 

     7. Consider that an excessive load can also cause most of the previously mentioned problems, or may be the original cause of the failure.

 

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